The present invention relates to a liquid crystal element and in particular relates to a method for driving a liquid crystal element employing a ferroelectric liquid crystal.
As examples of ferroelectric liquid crystals are known liquid crystals exhibiting chiral smectic C-phase (Sm*C) and chiral smectic H-phase (Sm*C) as shown in Table 1.
TABLE 1 ______________________________________ (n: integer) ______________________________________ ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ______________________________________
States of these ferroelectric liquid crystal molecules when subjected to an electric field are described in Neol A. Clark et al: "Submicrosecond bistable electro-optic switching in liquid crystals", Appl. Phys. Lett. Vol. 36, No. 11, June 1980, p.p. 899 to 901, for example, FIG. 1a to FIG. 1c shows these states.
As shown in FIG. 1b, when an electric field E is not applied, ferroelectric liquid crystal molecules 1 are helically oriented at an angle .theta. to the axis of helix 2. The angle .theta. is 20.degree. to 25.degree., for example, in the case of DOBAMBC.
As shown in FIG. 1a, when an electric field E exceeding the threshold electric field E.sub.C is applied to the ferroelectric liquid crystal molecules 1 thus oriented, the molecules 1 are aligned on a plane perpendicular to the direction of the electric field E with each long molecular axis having an angle .theta. with respect to the helix axis 2. When the polarity of the electric field E is reversed as shown in FIG. 1c, the ferroelectric liquid crystal molecules 1 are reversely aligned on the plane perpendicular to the direction of the electric field E with each long molecular axis having an angle .theta. to the helix axis 2.
This phenomenon takes place as fast speed. It is known that ferroelectric liquid crystal molecules may respond to a voltage pulse having a pulse width in the order of microsecond if an electric field of sufficient magnitude is applied to the molecules. Accordingly, it is expected to use ferroelectric liquid crystals to a large-sized display having a number of pixels (picture elements), optical shutter, polarizer and so on. Heretofore, however, the relationship between applied voltage and light transmitting state has not been made clear. In addition, a practical voltage suitable to drive the ferroelectric liquid crystals was also unclear.